(Top) - This is the first image ever taken of bright aurorae at
Saturn's northern and southern poles, as seen in far ultraviolet light
by the Wide Field and Planetary Camera 2 aboard NASA's Hubble Space
Telescope. Hubble resolves a luminous, circular band centered on the
north pole, where an enormous auroral curtain rises as far as 1,200
miles (2,000 kilometers) above the cloudtops. This curtain changed
rapidly in brightness and extent over the two hour period of our HST
observations, though the brightest emissions remained at a position
fixed in sun angle, near "dawn" in the north auroral band. The image
was taken on October 9, 1994, when Saturn was at a distance of 831
million miles (1.3 billion kilometers) from Earth.

The aurora is produced as trapped charged particles precipitating from
the magnetosphere collide with atmospheric gases -- molecular and
atomic hydrogen in Saturn's case. As a result of the bombardment,
Saturn's gases glow at far-ultraviolet wavelengths (110-160 nanometers)
which are absorbed by the Earth's atmosphere, and so can only be
observed from space-based telescopes. Saturn's magnetic field is nearly
perfectly aligned with the planet's rotation, giving the auroral "ring"
its symmetry centered on the pole. (The southern aurora is faintly
visible in this view despite the fact that Saturn's northern pole is
now tilted slightly toward Earth.)

The Hubble images demonstrate our capability to record from the Earth
the auroral brightness and distribution about Saturn's poles, which
will ultimately complement the in situ measurements of Saturn's
magnetic field and charged particles to be made by the NASA/ESA Cassini
spacecraft near the turn of the century.

Study of the aurora on Saturn had its beginnings a few decades ago.
The Pioneer 11 probe observed a far-ultraviolet brightening on Saturn's
poles in 1979. Beginning in 1980, a series of spectroscopic
observations by the International Ultraviolet Explorer (IUE) have
sporadically detected emissions from Saturn's auroral zones. The
Saturn flybys of the Voyager 1 and 2 spacecraft, in the early 1980s,
found auroral emissions confined to a circumpolar ring.

(Bottom) - For comparison, this is a visible-light color composite
image of Saturn as seen by Hubble on December 1, 1994. Unlike the
ultraviolet image, Saturn's familiar atmospheric belts and zones are
clearly seen. The lower cloud deck is not visible at UV wavelengths
because sunlight is reflected from higher in the atmosphere.